Fail-Safe Electromagnetic Motor Brakes

Lyndon B. Johnson Space Center, Houston, Texas

Fail-safe electromagnetic motor brakes are undergoing development for use in joints of robot arms. The brakes are needed for both static holding and emergency stopping. In the present context, "fail-safe" signifies that a brake disengages (that is, allows motion) when electric power to the brake is turned on and engages (restrains against motion) when the power is turned off. These fail-safe brakes are intended to replace commercial off-the-shelf (COTS) electromagnetic brakes that are large, complex, power-hungry, and hot-running.

The Brake Is Normally Engaged by spring-loading of the two brake pads against each other. When power is supplied to the electromagnet, the nonrotating brake pad is pulled away from the motor-shaft brake pad.

A fail-safe brake of this type (see figure) includes a brake pad that is attached to and rotates with a motor shaft, and a nonrotating brake pad in the form of a ferromagnetic plate. An electromagnet collinear with the motor and the brake pads is positioned a short distance from the ferromagnetic plate. When power is not supplied to the electromagnet, a torsionally stiff compression spring pushes the nonrotating brake pad axially into contact with the rotating one. The friction between the two brake pads effects the braking action; in other words, the brake is engaged. When sufficient power is supplied to the electromagnet, the nonrotating brake pad is pulled axially away from the rotating brake pad, thereby disengaging the brake. The spring offers an ancillary benefit in that its small torsional compliance absorbs torsional impulses (caused, for example, by unintended impacts of the robot against external objects), thereby helping to prevent damage to vulnerable motor-drive components.

The basic design concept has been proven in tests on a lifting-electromagnet model. Alternative fail-safe brake designs have been analyzed by use of a mathematical model of the electromagnet. Input design parameters include those of the material, shape, and size of the electromagnet core; the size and the number of turns of the electromagnet wire; and the supply voltage. Outputs of the model include the electromagnet current, power consumption, magnetic-flux density, and force on the ferromagnetic plate.

This analysis has led to a tentative optimum design for a fail-safe brake for a specific application in which the brake is required to hold against a torque of 0.5 lb-ft (≈0.68 N-m). The power required to disengage this brake would lie between 0.025 and 0.040 W. In contrast, the COTS brake that this brake is intended to replace must be supplied with a power of 8 W (at least 200 times as much power) to keep it engaged. Under a typical operational scenario, this comparison translates to much lower time-averaged power consumption for the present design. As an additional benefit stemming from lower power consumption, the fail-safe brake would run cooler than the COTS brake does.

This work was done by James David Jochim of Johnson Space Center.

MSC-23067